Hemi-Facial Spasm
This patient is undergoing a R/L retrosigmoid craniotomy microvascular decompression of the facial nerve for relief of hemi-facial spasm. The abnormal lateral spread facial EMG response was monitored to confirm adequate decompression of the facial nerve. The brainstem auditory evoked response (BAER) was also monitored to detect any compromise of auditory function with cerebellar retraction.
ephaptic (electrical impulse without a neurotransmitter) EMG responses to facial nerve stimulation
ABR monitored to detect any compromise in cochlear nerve function with retraction, etc.
sterile needle electrodes placed in the frontalis (temporal branch), orbicularis oculi (zygomatic branch), orbicularis oris (buccal branch), mentalis (marginal mandibular branch). electrodes also placed at the marginal mandibular and zygomatic for stimulation
cranial nerve dysfunction is caused by compression of a vascular loop and can be treated by moving the offending vessel off the nerve and placing a soft cushion between it and the nerve
abnormal muscle response, muscles innervated by one branch of the facial nerve respond when another branch is stimulated. This response is due to abnormal spread of activity from one branch on one branch of the facial nerve to another on the affected side. Since it is not suppressed by anesthesia, it can be recorded as long as the patient is not paralyzed.
For a typical procedure, recording electrodes would be inserted into the mentalis muscle (innervated by the marginal mandibular branch of the facial nerve VII) and the orbicularis oculi (temporal branch). Subdermal needle electrodes are also inserted adjacent to the marginal mandibular and temporal branches of VII for stimulation. Note that the stimulation voltage required will be higher (4-20V) than for direct stimulation of the nerve intracranially.
Offending vessel is usually PICA however in this case it is the Vertebral Artery
100 microvolts per division with 5 ms per division
looking for recordable activity in the mentalis muscle (lateral spread) with a latency of approximately 10ms that lasted until the facial nerve is decompressed. in some patient's, the autoexcitation disappeared after the dura was incised or when the archnoid was opened
It has been shown that when one branch of the facial nerve is stimulated, a response can be recorded from muscles innervated by other branches of the facial nerve. It is presumed that this is due to cross-transmission of the antidromic activity in the branch of the facial nerve that is stimulated.
It has been shown that it is stimulation of the facial nerve and not the trigeminal nerve.
Sometimes more than one vessel with a second one being subtle
stimulating electrodes set to deliver rectangular pulses of 150 usec duration at 2 to 8 pulses per second
response observed usually followed by shorter or longer after discharges of activity and sometimes spasms
new paper says: 3-5 stimuli, 0.5ms duration with 2 or 4 ms of interstimulus interval, train repetition of 2Hz and intensity up to 180mA
anesthesia: no muscle relaxants
warnings: (a) delay in latency of peak V of ABR of 0.4 ms considered to be a ‘watching signal’, hearing function
within safe limits; (b) delay in latency of peak V of ABR of 0.6 ms considered as ‘warning of real risk’; and
(c) delay in latency of peak V of 1 ms considered to be a ‘critical warning’. That delay of latency of peak V will produce hearing loss. Therefore, the surgeon is advised and he/she has to identify the cause of this delay in order to improve the responses (i.e., replace the retractor of the cerebellum).
MOTOR DIVISION OF THE FIFTH CRANIAL NERVE AT THE MASSETER
If the abnormal response is not seen at the outset, it can generally be
triggered by a brief train of stimuli at a high frequency (50 Hz). The amplitude of
the abnormal response is typically lower than that of the normal response, and
may drop even further after opening the dura, presumably because of a shift in
the relation of the vessel to the nerve. Nevertheless, an abnormal response at
some amplitude can generally be seen until the nerve is decompressed;
Stimulation: The marginal mandibular and zygomatic
branches are stimulated alternatively by a twisted pair of
subdermal needle electrodes (Nicolet Co., Madison WI,
USA). We used a single constant current stimulus of 0.2-
ms duration, at a stimulating rate of 1 Hz and threshold
intensity for eliciting lateral spread. During MVD, the thresh-
old intensity is adjusted for continuously eliciting the lateral
spread. If LSR disappeared, the intensity was increased up to
50 mA for few seconds in order to confirm the permanent
LSR disappearance (Møller and Jannetta, 1986a,b).
(b) Recording: Identical twisted pair needle electrodes as used
for stimulation are placed into the corresponding muscles
(orbicularis oculi and mentalis) for recordings.
Due to the fact that LSR disappears instantly in most of the pa-
tients when the offending vessel is moved off the facial nerve
(Fig. 1), monitoring the abnormal muscle response can guide the
surgeon during MVD which results in a better postoperative out-
come (Møller, 1987). If LSR only decreases in amplitude when a
vessel is moved off the facial nerve, it might be an indication thatanother vessel is also affecting the facial nerve. When this other
vessel is identified and moved off the facial nerve, the LSR disap-
pears completely.
Also may use Blink Reflex: See Blink Reflex
ephaptic (electrical impulse without a neurotransmitter) EMG responses to facial nerve stimulation
ABR monitored to detect any compromise in cochlear nerve function with retraction, etc.
sterile needle electrodes placed in the frontalis (temporal branch), orbicularis oculi (zygomatic branch), orbicularis oris (buccal branch), mentalis (marginal mandibular branch). electrodes also placed at the marginal mandibular and zygomatic for stimulation
cranial nerve dysfunction is caused by compression of a vascular loop and can be treated by moving the offending vessel off the nerve and placing a soft cushion between it and the nerve
abnormal muscle response, muscles innervated by one branch of the facial nerve respond when another branch is stimulated. This response is due to abnormal spread of activity from one branch on one branch of the facial nerve to another on the affected side. Since it is not suppressed by anesthesia, it can be recorded as long as the patient is not paralyzed.
For a typical procedure, recording electrodes would be inserted into the mentalis muscle (innervated by the marginal mandibular branch of the facial nerve VII) and the orbicularis oculi (temporal branch). Subdermal needle electrodes are also inserted adjacent to the marginal mandibular and temporal branches of VII for stimulation. Note that the stimulation voltage required will be higher (4-20V) than for direct stimulation of the nerve intracranially.
Offending vessel is usually PICA however in this case it is the Vertebral Artery
100 microvolts per division with 5 ms per division
looking for recordable activity in the mentalis muscle (lateral spread) with a latency of approximately 10ms that lasted until the facial nerve is decompressed. in some patient's, the autoexcitation disappeared after the dura was incised or when the archnoid was opened
It has been shown that when one branch of the facial nerve is stimulated, a response can be recorded from muscles innervated by other branches of the facial nerve. It is presumed that this is due to cross-transmission of the antidromic activity in the branch of the facial nerve that is stimulated.
It has been shown that it is stimulation of the facial nerve and not the trigeminal nerve.
Sometimes more than one vessel with a second one being subtle
stimulating electrodes set to deliver rectangular pulses of 150 usec duration at 2 to 8 pulses per second
response observed usually followed by shorter or longer after discharges of activity and sometimes spasms
new paper says: 3-5 stimuli, 0.5ms duration with 2 or 4 ms of interstimulus interval, train repetition of 2Hz and intensity up to 180mA
anesthesia: no muscle relaxants
warnings: (a) delay in latency of peak V of ABR of 0.4 ms considered to be a ‘watching signal’, hearing function
within safe limits; (b) delay in latency of peak V of ABR of 0.6 ms considered as ‘warning of real risk’; and
(c) delay in latency of peak V of 1 ms considered to be a ‘critical warning’. That delay of latency of peak V will produce hearing loss. Therefore, the surgeon is advised and he/she has to identify the cause of this delay in order to improve the responses (i.e., replace the retractor of the cerebellum).
MOTOR DIVISION OF THE FIFTH CRANIAL NERVE AT THE MASSETER
If the abnormal response is not seen at the outset, it can generally be
triggered by a brief train of stimuli at a high frequency (50 Hz). The amplitude of
the abnormal response is typically lower than that of the normal response, and
may drop even further after opening the dura, presumably because of a shift in
the relation of the vessel to the nerve. Nevertheless, an abnormal response at
some amplitude can generally be seen until the nerve is decompressed;
Stimulation: The marginal mandibular and zygomatic
branches are stimulated alternatively by a twisted pair of
subdermal needle electrodes (Nicolet Co., Madison WI,
USA). We used a single constant current stimulus of 0.2-
ms duration, at a stimulating rate of 1 Hz and threshold
intensity for eliciting lateral spread. During MVD, the thresh-
old intensity is adjusted for continuously eliciting the lateral
spread. If LSR disappeared, the intensity was increased up to
50 mA for few seconds in order to confirm the permanent
LSR disappearance (Møller and Jannetta, 1986a,b).
(b) Recording: Identical twisted pair needle electrodes as used
for stimulation are placed into the corresponding muscles
(orbicularis oculi and mentalis) for recordings.
Due to the fact that LSR disappears instantly in most of the pa-
tients when the offending vessel is moved off the facial nerve
(Fig. 1), monitoring the abnormal muscle response can guide the
surgeon during MVD which results in a better postoperative out-
come (Møller, 1987). If LSR only decreases in amplitude when a
vessel is moved off the facial nerve, it might be an indication thatanother vessel is also affecting the facial nerve. When this other
vessel is identified and moved off the facial nerve, the LSR disap-
pears completely.
Also may use Blink Reflex: See Blink Reflex
- left or right
- how long is the history of hemi-facial spasm
- where is the focus?
- what activities are interrupted
- have there been any interventions; botox?
- is there any weakness, numbness or any other facial nerve defects
- MRI shows clear vascular loop in the cerebellopontine angle?
- is the patient negative for tumor?
- audiogram?
- tumor?
